1) Dave Davison at CH4D said he'd take the dies back if they weren't the right ones.
2) I did mike the bullet part of the casting. It mikes .257, and the casting looks just like the picture of the regular .257 Roberts in Richard Lee's Modern Reloading.
3) The rifle was made before 1930. Remington commercialized the cartridge and changed the taper of the neck from 15 degrees to 20 degrees in 1934. The seller said that regular .257 Roberts would not chamber and included a messed up .257 case which had not chambered.
I think the odds are pretty overwhelming that the chambering is going to be the old .257 Neidner wildcat.
I would have preferred to have just gotten the chambering modified so I could buy ammo, but I asked for opinions on several gun discussion boards and got about 20+ replies telling me that it would be sacrilege to change the chambering. Not one reply was in favor of modifying the chambering. Sigh.
